Sunday, 11 September 2016

Weekend Plot: update on WIMPs

There's been a lot of discussion on this blog about the LHC not finding new physics. I should however give justice to other experiments that also don't find new physics, often in a spectacular way. One area where this is happening is direct detection of WIMP dark matter. This weekend plot summarizes the current limits on the spin-independent scattering cross-section of dark matter particles on nucleons:

For large WIMP masses, currently the most succesful detection technology is to fill up a tank with a ton of liquid xenon and wait for a passing dark matter particle to knock one of the nuclei. Recently, we have had updates from two such experiments: LUX in the US, and PandaX in China, whose limits now cut below zeptobarn cross sections (1 zb = 10^-9 pb = 10^-45 cm^2). These two experiments are currently going head-to-head, but Panda, being larger, will ultimately overtake LUX. Soon, however, it'll have to face a new fierce competitor: the XENON1T experiment, and the plot will have to be updated next year. Fortunately, we won't need to be learning another prefix soon. Once yoctobarn sensitivity is achieved by the experiments, we will hit the neutrino floor: the non-reducible background from solar and atmospheric neutrinos (gray area at the bottom of the plot). This will make detecting a dark matter signal much more challenging, and will certainly slow down the progress for WIMP masses larger than ~5 GeV. For lower masses, the distance to the floor remains large. Xenon detectors lose their steam there, and another technology is needed, like germanium detectors of CDMS and CDEX, or CaWO4 crystals of CRESST. Also on this front important progress is expected soon.

What does the theory say about when we will find dark matter? It is perfectly viable that the discovery is waiting for us just behind the corner in the remaining space above the neutrino floor, but currently there's no strong theoretical hints in favor of that possibility. Usually, dark matter experiments advertise that they're just beginning to explore the interesting parameter space predicted by theory models.This is not quite correct. If the WIMP were true to its name, that is to say if it was interacting via the weak force (meaning, coupled to Z with order 1 strength), it would have order 10 fb scattering cross section on neutrons. Unfortunately, that natural possibility was excluded in the previous century. Years of experimental progress have shown that the WIMPs, if they exist, must be interacting super-weakly with matter. For example, for a 100 GeV fermionic dark matter with the vector coupling g to the Z boson, the current limits imply g ≲ 10^-4. The coupling can be larger if the Higgs boson is the mediator of interactions between the dark and visible worlds, as the Higgs already couples very weakly to nucleons. This construction is, arguably, the most plausible one currently probed by direct detection experiments. For a scalar dark matter particle X with mass 0.1-1 TeV coupled to the Higgs via the interaction λ v h |X|^2 the experiments are currently probing the coupling λ in the 0.01-1 ballpark. In general, there's no theoretical lower limit on the dark matter coupling to nucleons. Nevertheless, the weak coupling implied by direct detection limits creates some tension for the thermal production paradigm, which requires a weak (that is order picobarn) annihilation cross section for dark matter particles. This tension needs to be resolved by more complicated model building, e.g. by arranging for resonant annihilation or for co-annihilation.

Here is some exciting news for those interested in the increasingly relevant and empirically supported idea that stellar-mass black holes comprise the dark matter that has confused theoretical physicists and eluded experimental physicists for over 50 years.

A research team has just discovered that a globular cluster of stars appears to contain on the order of 100 stellar-mass black holes instead of the expected few (Pueten et al, Mon. Not. Royal Astr. Soc., 2016).

Every galaxy in the cosmos has roughly between 10 and 1,000 globular clusters associated with it. If a significant fraction of those GCs has roughly 10 - 100 black holes, then that is a truly enormous number of previously undiscovered black holes, and would account for enough mass to make up the entire dark matter, especially when combined with the stellar-mass black hole populations already discovered by the MACHO and LIGO research groups.

Robert, if you do the arithmetic, even a million extra stellar mass black holes per galaxy doesn't even tip the scale significantly. MACHOS are basically excluded from consideration across the board. It's long past time to find another horse to beat.

Jester, is there a possibility that dark matter is an entirely different sector of particle physics that does not interact with known forces at all? Would such an option be constrained by any considerations?

Xezlec, Anonymous, mfb -- The data analyzed by CMS and Atlas are both delivered by LHC. Therefore LHC has already delivered above 30 fb^-1 of data at 13 TeV (including 2015 and 2016), about half of which has been analyzed by CMS and the other half by Atlas, and presented at ICHEP 2016.

In this 30 fb^-1 of data delivered by LHC (@13 TeV) that has been analyzed and presented, there is no discovery of SUSY.

Non only Xenon. There is also Argon. The DEAP detector at SNOLab is just starting taking physics data and shouldreach 10-46 at 100GeV. Anyway, the floor is still far and we have to wait the LZ detector in the next decade.I'm looking forward to lighter candidates for DM though...K.

Robert, pasting your comments below every possible post contributes to the perception of black hole dark matter as a crackpot idea, which is unfair. This discussion has nothing to do with the topic of this post, so it's better to stop it at this point.

RBS, yes, it is perfectly possible that DM interacts with us only gravitationally (it would have to be a weird accident that the cosmic abundances of dark and visible matter are comparable, given the two don't talk to each other, but then that is a weird accident anyway in most models where they do interact...)

"Once yoctobarn sensitivity is achieved by the experiments, we will hit the neutrino floor: the non-reducible background from solar and atmospheric neutrinos. This will make detecting a dark matter signal much more challenging..."

The underlying assumption is that DM couples super-weakly with nucleons through a mechanism that mimics standard interaction via gauge or Yukawa charges. But what if this assumption is invalid to begin with? For example, what if DM represents an "exotic" spacetime condensate whose topology and properties do not fit the traditional model of SM interactions?

Sure, these constraints are valid only under very specific assumptions, which may or may not be correct. There are tons of perfectly valid dark matter models that cannot be probed by direct detection. Axions are one prominent example, but you can even write down a WIMP model where the cross section is too low to ever be detected.

Even if direct detection is close to impossible, indirect detection means need to be developed to confirm or falsify DM models. There is no other way around if we wish to get to the bottom of the DM puzzle.

With WIMPs, SUSYs, axions, keV sterile neutrinos and WHYs (What-Have-You-s) out of sight, it may already be timely to look back at the ruled-out neutrinos as the source of cluster DM. They do exist and do explain very well the lensing properties of the galaxy cluster A1689. Their mass has to be some 1.86 eV and they need to have Dirac signature, so also sterile sisters of the same mass. However, their problematic performance for the BBN, CMB and structure formation has to be explained.As to galactic DM, it was discussed recently by Hawkins that MACHOs are not properly ruled out (because they were fit to models with a overly heavy Galactic halo), while the observation of several free floating planets may point at a large family of such objects.

Neutrinos were once out of sight, and are now in plain sight, and so your extrapolation of the concept of analysis, prediction and discovery of new fundamental particles to axions, which extremely well motivated and well studied SM fields and particles, possesses little or no credibility whatsoever. In fact, looking at the best guess mass from QCD lattice approximations of topological susceptibility, they appear to be right on the verge of discovery, and furthermore, as an excitable BEC - they also appear easily manipulated. Looking at axion Higgs dynamics as known from condensed matter physics, interesting effects can be predicted well in advance of their discovery.

"If the WIMP were true to its name, that is to say if it was charged under weak interactions, it would have order 10 fb scattering cross section on neutrons. Unfortunately, that natural possibility was excluded in the previous century."

I keep hearing people say this--even people who work on dark matter experiments--and I think it's important to state it correctly. If dark matter were charged under weak interactions and chiral, receiving all of its mass from the Higgs mechanism, yes, it was excluded by the failure to observe Z-mediated scattering. But the formulation you've written is false, as the example of an SU(2) triplet illustrates. For many particles charged under weak interactions the dominant direct detection process could arise at 1-loop, safely below the current bounds.

So the only silver bullet approach to detecting DM as alternative to model-guessing would be gravity related. We are in a galaxy and some density of DM has to be right around us. Could there be some sensitive dynamics experiment in which we could detect and probe its properties, either on Earth or in its vicinity? E.g. if we could estimate the mass of ordinary matter (and energy) around Earth then any delta visible from precise dynamics observations would have to be attributed to DM, right?

Matt I agree it was sloppy writing. What I meant there is not "charged under weak interactions" but rather "experience the weak force" i.e. "couples to Z with O(1) strength". I'll re-formulate it better.

Even better: we know there're super massive black holes in galaxies. We know that there's dark matter in the galaxies and a lot of it, more than the ordinary matter. If it were anything like our normal, regular matter we would have to see something as it accelerates and falls beyond the horizon. But there's nothing there, extra or out of order. Doesn't this tell us something? If we don't see even light, if the dark light itself that must pervade this Universe avoids us, what is the chance what we would see some other sort of interaction with our forces? Of course we should keep trying testing what we can. But can we count on something to be found there, eventually?

And then - who said there has to be only one DM sector and not six, ten, a million united only at the Planck scale and linked together in the shared space only by gravity? There can be a population of "standard models" each with its own ensemble of particles right around us and we just happen to reside in a sizable one - but by no means the smallest? Would it contradict something - and maybe there would be something we could derive from such models that could be tested?

@Anonymous 12 September 2016 at 15:03: Thanks for trying to explain the machine I work on to me, but you are wrong. The delivered 30/fb as of today (mentioned by Xezlec) are per experiment(ATLAS and CMS, plus 1.5/fb for LHCb and 11/pb for ALICE). The studies ATLAS and CMS showed at ICHEP were based on 10-15/fb per experiment, at most half the dataset the experiments have now.

Re: RBS, I prefer dancing unicorns in the flame duct myself. What most credible investigators are doing now, given a tentative null result of new particles and physics up into the TeV range, is replacing wild speculation with good motivation.

meV scale axions are not only well motivated, couple to gravitation in a one to one manner, and axion dynamics and the Higgs mechanism are both showing up everywhere in condensed matter physics theory and experiment. You should have enough to work with.

Crank ideas may be useful, but they are rarely as well motivated as axions seem to be.

Hi mfb -- It is up to Jester and Lubos how they interpret their bet which is based on discovering SUSY in first 30 fb^-1 data delivered by LHC at 13 TeV.

All I am saying is that 30 fb^-1 of LHC data at 13 TeV has already been analyzed by the end of ICHEP 2016 -- about 3-4 fb^-1 each by CMS and Atlas in 2015 and about 13 fb^-1 or so each in 2016. According to my calculation, total data delivered by LHC is what is delivered to (CMS + Atlas) where I am neglecting the much smaller data to LHCb.

Of course more data has accrued and now it is nearing 30 fb^-1 of data in each CMS and Atlas....so total 60 fb^-1 of data has been delivered by LHC.

Basically CMS and Atlas analyze different data sets, and not the same data as you know.One can do a joint analysis of both data sets, as is often done.

By "supersymmetry found", we could mean that e.g. after 30 inverse femtobarns of data, there will exist at least one paper by ATLAS, CMS, or some combination of them that will claim, in the title or abstract, that they found evidence for supersymmetry and/or superpartners, or that they will have a careful interpretation that "it might be SUSY/superpartner" and there will exist at least one phenomenology paper with at least 10 citations that will claim that SUSY found is the most likely interpretation of the data

[note the words "combination of them"]

On the other hand, you're [Jester] the winner after 30 inverse femtobarns (at 14 TeV, or more than 12 TeV) if no "discovery paper" of this kind is in the waiting line. OK? If there is a paper waiting at that moment, we wait for 1 year to decide.

For more on the bet see comments section of 2008 post: http://resonaances.blogspot.com/2008/09/what-will-lhc-discover.html

There was lots of good theoretical motivation for dark matter particles with cross-sections of interactions with nucleons on the same order of magnitude as neutrinos, most notably, from SUSY theories. But, this parameter space is almost completely ruled out.

The notion of "super-weakly" interacting massive dark matter particles is in my view far, far less well motivated theoretically than dark matter particles which simply have no couplings whatsoever under any of the three Standard Model forces, but could conceivably have self-interactions via some new force limited to the dark sector in addition to gravity. All this takes are particles that are exclusively "right handed" which is hardly a wild and crazy idea when were already have a class of particles (neutrinos) that are exclusively "left handed." One of the best established properties of the weak force is that weak force decays are "democratic" treating every possibility that is not conservation of mass-energy prohibited equally, and there is just no way to shoehorn any sub-45 GeV "super-weakly" interacting particle into that aspect of the weak force.

It is also very hard to come up with any kind of thermal relic dark matter candidate of significantly more than keV WDM mass that reproduces observed dark matter phenomena from astronomy without a self-interaction term, which for practical purposes means that CDM theories need both a dark fermion and a dark boson carrying a new dark force (which is really the worst of both worlds - a de facto gravity modification and particle dark matter, rather than one or the other). WDM models aren't ruled out yet, but are on the verge of being over constrained by the empirical evidence. Really, sterile neutrino-like WDM is by far the most plausible game in town for particle dark matter by a long shot, and by definition, we will never be able to directly detect it in direct detection experiments (even if they break the neutrino noise threshold) or at any collider ever.

In particular, I've seen nothing in the astronomy papers for many years that even seriously attempts to reconcile a MACHO hypothesis such as primordial black hole dark matter with the inferred dark matter halo distributions derived from galaxy and galaxy cluster dynamics. Nobody even tries to run MACHO dark matter simulations that reproduce reality because people learned long ago that this doesn't work. And, my Baysean priors based on recent data seem to show that black holes with significant electromagnetic fields (which are inconsistent with dark matter) are the norm and not the exception.

The wiggle room in our measurements of the behavior of the Higgs boson for it to have any non-SM interactions (such as DM portal interactions) grows significantly smaller with each passing season, and there is every reason to be highly skeptical of it. Maybe there is another Higgs boson that interacts solely with the dark sector, but it is only a matter of time before an absence of missing energy in Higgs decays rules out Higgs portal dark matter, by which I mean two or three years tops.

Theory space is not very well developed for non-thermal relic dark matter models, with the axion providing pretty much the only game in town on that score so far. But, the notion that the absence of CP violation in the strong force needs any more explanation than Nature set that constant to zero, seems to me to be as misguided as the last couple of decade's obsession with "naturalness" which has done us no good whatsoever. So, I don't see the axion as well motivated and I'll consider other non-thermal dark matter models when somebody actually proposes them.

Many papers on MACHOs and their possible contribution to the dark matter have been published over the last 10 years.

Just as one example, he might want to study carefully the paper:Primordial Black Holes as Dark MatterB. Carr et al.Submitted to arxiv on 8 Aug 2016

Then there are Kashlinsky's recent papers, and those by the Johns Hopkins group and those by the Japanese group.

It is generally admitted, although somewhat reluctantly, by those who have adequate knowledge of dark matter research and no strong bias, that MACHOs could contribute up to 10-20% of the dark matter. This would involve up to 100,000,000,000 stellar-mass MACHOs.

The standard follow-up line is that the above number is not 100% of the dark matter, so there is little interest in MACHOs as dark matter. This conclusion has always seemed a bit bizarre to me.

If you don't consider axions, axion physics, axion electrodynamics, gravitational axions and BECs as 'motivated' then clearly you have not been perusing the condensed matter physics section of the ArXiv.

Why don't wee see ANY visible signature from DM participating in super high mass astronomic events? Is this because of some sophisticated dynamics, or simply because whatever is emitted by these interactions cannot be detected because it doesn't interact with us even in the faintest way?

I'd like to follow up with a more moderate perspective on the topic of DM.

In my opinion, all of you, Andrew included, raise very good points in this debate. There is no right and wrong at this point and I think many scenarios are certainly plausible. We all have biases, but it is a mistake to jump the gun and exclude axions, sterile neutrinos, MACHOS or any other exotic solutions on the DM puzzle.

Re: RBS - Axions may come in flavors, but traditionally PQ axion photon coupling is inversely proportional to the electroweak symmetry breaking scale f_A which in a flat universe is somewhere roughly half way to Planck scale physics. Therefore the axion coupling to photons is extremely weak, although not undetectable, but currently they are invisible. The experimental detection challenges are roughly on par with that of gravitational waves, or historically on par with neutrino detection. Therefore many think they are on the verge of discovery if their mass lies in the sub meV range, which is what lattice QCD calculations of QCD topological susceptibility reveals.

Basic questions amenable to speculation and modelling are many. Are PQ axions the only axion flavor, is the universe really flat at that energy scale or is there some kind of coupling to gravitons and quarks, and is gravitational black hole collapse a quantum critical process, etc. Speculation and modelling are all over the map, but what is not ambiguous is that axion physics and the Higgs mechanism are universal. Both of these concepts, including strong coupling and high density quantum criticality, show up just about anywhere one looks, in just about every domain of physics under consideration. That's a big hint that this problem, the dark matter problem, is solvable. Dark energy seems dependent upon dark matter.

Back to the subject at hand, MACHOS, the exclusions are moving in the opposite direction, that is, pending future LIGO data, they stand at the verge of refutation. A space based LIGO detector would go a long way to solving this problem.

The only sure thing here is that the end of physics is not nigh, unless MACHOS constitute all of dark matter. Then I would foresee definite problems with continuing progress in physics, at least outside of condensed matter physics.

@ andrew Nobody even tries to run MACHO dark matter simulations that reproduce reality because people learned long ago that this doesn't work.

This community excels in suppressing works on ideas that are considered as ruled out. But now there is doubt on whether Machos are properly ruled out. Maybe the Gaia data give clues. As to cosmological DM simulations, they work with cells of a million solar masses.

'By "supersymmetry found", we could mean that e.g. after 30 inverse femtobarns of data'

Written that way, i'm quite sure it means 30/fb per experiment. A combination (which doesn't happen if both experiments expect to double their dataset quickly, especially if there is something unusual new) would then use a total of 60/fb. I also guess he meant analyzed, not collected. The phenomenology paper would also need some time to be written and collect citations.

Regarding the newest GAIA results, a bigger Galaxy means a bigger galactic halo, and that may mean considerably more MACHO dark matter candidates.

One anonymous poster opined that the discovery that MACHOs constitute the dark matter would be "the end" of physics. This is too pessimistic. I think it might be a long-needed reboot of physics - a beginning rather than and end.

Actually no, Robert, that's not what I said. What I implied was that if LIGO were to suggest that intermediate black holes constituted the entirety of dark matter, there would be problems in progress in physics, since that would imply that space was flat up to Planck scale energy. I myself do not believe this to be the case, first because recent results have basically excluded primordial black holes as any major constituent of dark matter, and on the contrary condensed matter physics theory and experiments are already validating axion - Higgs physics. Furthermore several routes to curvature simulation have already been demonstrated in the laboratory. And quantum criticality is universally accepted as tied into scaling divergence and singularities.

I have my own crackpot hypothesis about what gravitational and QCD axions are all about, but this isn't the place to push my own preferred pet theory on how that works. The topic here is exclusion, and experimental verification of exclusion limits, specifically with galactic halo black holes. A simple scholar search will bring you up to date. I'm here because I am interested in the subject of glaring gaps of knowledge about the universe, and I still have an open mind on the problem.

Thus, I am interested in the state of the art and alternative opinions on this. Obviously my knowledge of this subject is woefully incomplete.

Well, one "anonymous" who favors axions said, and I quote: "The only sure thing here is that the end of physics is not nigh, unless MACHOS constitute all of dark matter."

So that anonymous actually did say that MACHO dark matter would be the end of physics .

More importantly, everyone is entitled to their personal beliefs and to select the literature that supports those beliefs. However, the final verdicts on our ideas, assumptions, models, theories and paradigms are handed down by nature via empirical results. The history of science is littered with the corpses of theories that were "too beautiful to fail". Or "highly compelling", "robust", etc.

Do we really need a space based LIGO for a verdict on LIGO style MACHO? If BH were to account for all the DM in our galaxy, where would be some tens of billions solar masses in MACHO dark matter, most of it near the galaxy core. Yet the mass of Saggitarius-A is only a few million SM and most if not all of it is accounted for by the regular star and gas dynamics. How would this fact be explained by MACHO DM theorists?

That is like claiming elementary magnetic monopoles have to exist just because they exist as quasiparticles in solid-state physics. What is next? Are we looking for lattice defects in spacetime and a melting temperature just because someone takes analogies too far?

Re: mfb Majorana fermions exist, both in the math and in the experiments. If anything the analogies haven't been taken far enough because black holes exist, quantum singulariies exist, dielectric catastrophes exist, and mathematics exists. Recent results in condensed matter physics confirms that over and over again, in just about every place one cares to look. So yes, defects in spacetime exist. Unfortunately they are hidden by event horizons that will be very difficult to defeat until microscopic black holes are produced in future particle accelerators, but certainly until those become available at sufficiently high energies, we can simulate them in the laboratory. I believe it's already been attempted. If you've got better ideas on how to solve these problems, I am open to your description of them here or elsewhere.

Re: Robert Axions are my preferred hypothesis at this point only because evidence for MACHOs is converging to refutation, whereas evidence for axions is converging to sub meV masses that are within the range of current detection techniques, and they are well motivated by theory and experiment. That being said, I expect the final answer on dark matter will be something unexpected. Nature always seems to deliver, regardless. What I do expect is that the solution to the dark matter/dark energy problem will involve some aspect of spacetime geometry and topology, microscopically at the (putative) graviton scale, mesoscopically at the axion scale, astronomically at the black hole scale/cosmic inflation scale. If that ends up meaning primordial black holes, then I am totally ok with that, since Jester has presented the evidence that there is some wiggle room left with that hypothesis.

Re: RBS It can't. I can only speak for myself, but both hypotheses, MACHOs and WIMPs are rapidly falling out of favor with the one person that counts - myself. On the other hand, SUSY, MACHOs and WIMPs may be right around the corner, and could be right around the corner for a long time. I'm not proficient in anything but the big picture. What I am interested in is verifiable and repeatable results and solutions. And you are correct, a space based LIGO detector is mostly likely not required to verify or rule out MACHOs. But given the LISA result, the Hubble, etc, the promise of space based observation on a large scale is very promising indeed. There are also supermassive black holes to consider as well.

The only thing I am absolutely convinced of is that the most useful results will come out of condensed matter physics laboratories and from cold atom optical trapping. I am most interested in quantum cooling techniques for the replacement and/of elimination of pulse tube and helium 3/4 dilution refrigeration for advanced detectors etc., which has great promise in reducing the cost of astronomical LIGO observations, particle accelerators and experimental simulations of exotic physics.

MRS Hawkins published a paper in the Monthly Notices of the Royal Society (2014, I think;available at arxiv) in which he argues that up to 100% of the Galactic dark matter could be in the form of MACHOs, given existing microlensing data and galactic models that are alternative to the conventionally assumed model (since questioned by new GAIA results), but are reasonable models.

Rumors that MACHOs could not be the dark matter are unfounded at present. There has been strong resistance to the possibility of stellar-mass black hole DM. Those rumors worked in the past but are increasingly untenable in the face of growing contradictory evidence.

Robert,how would one explain a simple fact that there'd be five times more invisible but regular matter floating around the galaxy core, but none of it is getting to the central super massive black hole?

I am perplexed by your question. Primordial black holes are by no means "regular" and in fact they might be Kerr-Newman black holes rather than Kerr black holes, which would make quite a difference in their behavior.

Also, the K2 mission is taking a closer look at the center of the Galaxy. Let's see what they observe before jumping to conclusions.

Also, whether something is drawn into the galactic center depends on its velocity and location so I am confused b what you mean to imply. The galactic halo does not get sucked into the SMBH at the center. Maybe you could refine your argument?

Contra MRS Hawkins (who is pretty much a lone voice in the wilderness) multiple, independent studies, using different methodologies that make their collective conclusions robust, severely constrain the MACHO hypothesis. For example:

"[A] recently discovered star cluster near the center of the ultra-faint dwarf galaxy Eridanus II . . . combine[s] with existing constraints from microlensing, wide binaries, and disk kinematics to rule out dark matter composed entirely of MACHOs from ~10^−7 M_sun up to arbitrarily high masses." Timothy D. Brandt, "Constraints on MACHO Dark Matter from Compact Stellar Systems in Ultra-Faint Dwarf Galaxies" http://arxiv.org/abs/1605.03665 The same exclusion range is concurred in by Andrea Addazi, Antonino Marciano, Stephon Alexander, "A Unified picture of Dark Matter and Dark Energy from Invisible QCD" http://arxiv.org/abs/1603.01853 who try to make a class for MACHO candidates below that mass threshold. And, the same lower mass bound constraint (with an upper bound of an exclusion range of 5 solar masses) was set forth in 2009. M. Moniez, "Review of results from EROS Microlensing search for Massive Compact Objects" http://arxiv.org/abs/0901.0985 The 10^-7 M Sun limitation acknowledged in all three papers is an upper bound of 2*10^23 kg which is about 1/3d the mass of the planet Mars.

Another 2014 analysis rules out MACHOs of more than 5 solar masses. Miguel A. Monroy-Rodríguez, Christine Allen "The end of the MACHO era- revisited: new limits on MACHO masses from halo wide binaries" http://arxiv.org/abs/1406.5169. Similarly, another paper rules out MACHOs larger than one solar mass and 0.5 solar mass white dwarfs concluding that they constitute "less than 10% [of dark matter] at the 95% confidence level." S. Torres, J. Camacho, J. Isern, E. Garcia-Berro "White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy" http://arxiv.org/abs/1001.1618 But, a 2010 survey pretty much rules out MACHOs of less than 10 solar masses as a source of more than 20% of dark matter, of less than solar mass as a source of more than 7% of dark matter, and of less than 0.1 solar masses, as a source of more than 4% of dark matter. L. Wyrzykowski, et al., "The OGLE View of Microlensing towards the Magellanic Clouds. III. Ruling out sub-solar MACHOs with the OGLE-III LMC data" http://arxiv.org/abs/1012.1154 Ruling out the rest of the parameter space.

@Robert L. Oldershaw, "MACHOs could contribute up to 10-20% of the dark matter." I don't disagree that there could be a significant amount of overlooked baryonic "dim matter" in the universe, including MACHOs and also baryonic matter in the form of interstellar gases and dust and dim stars obscured by light pollution from brighter objects in our line of sight to them. But, unless it is close to 100%, this means that there must still be some other "new physics" solution to the dark matter problem. All this does is double the amount of ordinary matter while reducing one for one the amount of dark matter in the lamdaCDM standard model of cosmology while still leaving us with a universe that is predominantly dark matter. The big picture remains the same. And, it is not just that there aren't enough MACHOs, the lamdaCDM model couldn't predict what is observed as well as it did if its assumptions about dark matter (i.e. that is has almost no non-gravitational interactions with other matter) weren't true, something that is certainly not true of any MACHO model.

There will rarely be a complete consensus on any recent conclusion, which is healthy in case holes in the dominant paradigm were missed. But, the exclusions relevant to MACHO dark matter have been extremely tight for many years and continues to grow more strict as new data comes in.

@Alex Small "Wouldn't sterile neutrinos be detectable through oscillation experiments?"

A true sterile neutrino might be detectable in that way at some point, although the heavier the sterile neutrino, the smaller the mixing angle would be and a sterile neutrino in the hundreds or thousands of eVs or more would have to have a mixing angle far too small to detect in current experiments to fit existing data. In a 4x4 PMNS matrix, the combined probability of an oscillation to such a heavy neutrino species (i.e. the sum of the magnitude of the square of the relevant components of the matrix) would be on the order of one in a billion or so, and is probably much lower than the probability of a top quark decaying to down quark rather than a bottom or strange quark in the Standard Model CKM matrix (something that I do not believe has never been definitively observed in any experiment to have actually happened).

Incidentally, the existing data are consistent with a three neutrino model at this point and rather strongly disfavor the 3+1 neutrino model proposed to explain reactor anomalies that have grown less statistically significant than they were when they were first noticed.

But, at least when I say "sterile neutrino-like" what I mean is a right handed fundamental particle with no weak force, strong force, or electromagnetic interactions (but with gravitational interactions proportionate to its mass-energy) that are either completely stable, or at least metastable on time frames comparable to the age of the universe. In other words, a particle with properties similar to a hypothetical right handed neutrino without necessarily actually being a neutrino that oscillates with other flavors of neutrinos. Such a "sterile neutrino-like" particle could arise in all sorts of models having nothing to do with true neutrinos.

It is completely unquestionable that a _fraction_ of dark matter is in the form of MACHOs -- for example, the continual discovery of new exoplanets shows this (yes, they are MACHO dark matter). It is also completely unquestionable that this fraction is a small one, as all the massive evidence from many sources since the 1980s, and even some before, has explicitly shown. This is just a silly discussion.

Axions are, indeed, where the betting money is going right now, for well-motivated reasons: since both of the currently-known classes of other well-motivated options have been constrained to the far corners of the closets that they may hide in.

Jeez, MACHOs cannot be baryonic and this is well-known to astrophysicists.

If we are talking about 100s of billions (and that's not the English version of billion) of MACHOs with masses on the order of 0.2 to 35 solar mass, then the most likely candidate objects are primordial black holes, which are non-baryonic. So many stellar-mass objects would conflict with observations if they were baryonic. Clearly they are not baryonic.

Many of the constraints on the size of the MACHO population are based upon very dubious assumptions, like the mass distribution is usually assumed to be a sharp delta function with a very limited mass range. This is almost certainly wrong! Then there are other assumptions that are less dubious but still inadequately untested, such as the Galactic model, the possible locations of highest incidence of MACHOs and the velocity distribution of the MACHOs.

The bottom line I keep trying to emphasize is that if we really want to solve the dark matter enigma, then we should keep an open mind and fight against biases that may be subverting our efforts.

We know that the dark matter clumps together on a large scale.For a dust cloud to collapse it needs to lose thermal energy by radiating infra red photons.In order to clump together the wimps need to lose thermal energy.If the wimps can radiate photons these would presumably be radio waves.Another mechanism would be for the wimps to exchange thermal energy with ordinary matter via gravitational interaction. Then the ordinary matter could radiate photons.

@Robert, wouldn't you have to assume that those primordial BH would have some strange distribution in the universe that is completely opposite to the regular matter - or with some new force of repulsion from it? What would be the reason? Wouldn't it be like piling one stretch upon another only for the sake of an argument? Anyways, other than BH dark matter of which much was said already or really sterile "neutrino" that can't be detected in principle, what are (are there still) plausible candidates for particle dark matter?

@RBS, no I would not have to make any of those dubious assumptions, although there is no reason to rule out the possibility that the observed Galactic distribution of PBHs extends beyond the Galactic halo and continues into intergalactic space. While high-velocity stars are observed to escape the Galaxy, it is possible that PBHs outnumber normal stars in this realm by larger ratios that the 5-to-one ratio estimated for intra-galactic distributions.

Only future observations can test the actual distribution in the observable universe.

Recent observations have cast doubt on the viability of sterile neutrinos. The results paper was posted to arXiv and discussed in Nature, Science, etc.

It looks like the experiment, barring last minute revelations every possibility of which we should duly explore is hinting more and more strongly with each stronger bound on the possibility that particle DM can be sterile in the sense of having no interactions with the standard matter either in theory or for all practical purposes and for a long time. Is this the time to acknowledge and begin exploration of such a possibility? Would there be any meaningful means and tools to probe it?

Concerning dark matter: Can someone comment on https://arxiv.org/abs/1609.05917 ("The Radial Acceleration Relation in Rotationally Supported Galaxies")? It looks odd, but I don't know enough about astrophysics to say more than that.

To me this means that the mass of the ordinary matter, the baryons, is strongly correlated and strongly coupled to the mass of the invisible dark matter. They move in lock step. Certainly this is a landmark result that eliminates decoupled and/or weakly coupled theories such as MACHOS, etc., Robert, sorry. Of course I am biased, but it also brings 'compositeness' (the fundamental structure and behavior of bosons and fermions) in condensed matter physics theories and experiments right back into the picture. Otherwise there is something that we still don't understand about gravitation, which seems unlikely to me. However MOND theories are still out there.

Combing through the discussion I saw some comments about axions being supported (?) by solid state physics (!). I can't say that I got it, though. Would someone be kind enough to strangers to give some links?

This isn't my blog and links tend to disappear here, but you can find what you need merely by searching 'axion condensed matter physics' or 'axion dense QCD'. I only ran across this a year ago, but reviewing the literature - it has a very long history.

All I can say is that in the year or so I have been working on it I have made some progress. Now that WIMPS and MACHOs are basically excluded, and it appears SUSY and exotic string/M type theories aren't available at any reasonable energy levels, if at all, this problem is poised to be solved. In fact, I can probably say that I (mayO have solved it. As with Einstein the trick is to find the right theory, but nowadays it's more a matter of exclusion rather than building it up from scratch. However, like general relativity it must also make contact with observation and experiment. 1609.05917 is the key observational evidence obviously, but that only really tells me what isn't happening here. To see what is happening one must resort to simulation and theoretical analogs in condensed matter and ultra cold quantum gas experiments, for instance, Martin Zwierlein's recent work on high temperature superconductivity. Ultimately then, we would like to detect an actual particle, but when dealing with gravity modification it might not be that simple. When I first encountered this, I thought it was all crackpot. You know, like special relativity.

Looks like dark matter puzzle will hold the answers to many questions while offering more opportunities to probe it should LHC experiments come short of expectations in delivering the "new physics". Should it perhaps move to the forefront of HEP research ahead of traditional accelerator and string physics?

@ Andrew "when I say "sterile neutrino-like" what I mean is a right handed fundamental particle with no weak force, strong force, or electromagnetic interactions (but with gravitational interactions proportionate to its mass-energy) that are either completely stable, or at least metastable on time frames comparable to the age of the universe."

Beyond the doom and gloom - what questions do we have to guide the research beyond SM? Would they some day lead us to a higher theory than the SM?

1. Dark matter2. Neutrinos - mass generation and how are they related to SM3. GUT and generations - is/why are there only 3 generations? Why and how are the strong forces aligned with the electroweak ones and the neutrino sector?4. Entanglement, event horizon and quantum gravity - are there any manifestations of QG that can be investigated at this stage and guide further research?

There's a very long list e.g at <a href="https://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics>Unresolved problems in physics</a> but from a more narrow point of practical HEP research a newly entering THEP student could work on in the coming years of late and post LHC?

Less than 3 sigma, in an obscure channel where I still would expect that the collaborations had a look at it?

If this is Z->bb+X with X->mu mu, where is the Dalitz plot? Where is the pT distribution of this potential X?

The fit in figure 24 with the dielectron spectrum has a chi^2/ndf of 0.39 with 21 data points. 7 fit parameter means 14 degrees of freedom, leading to p=0.9783. Looks odd (overfitting?).

Where is a check if this could be more like Z->(b mu) + (b mu), especially as at least one jet/muon angle tends to be small (figure 15 a)? Why is there not a single event where the muons are well separated from the jets? Where is a toy prediction for this suggested particle X? Why is at least one jet broader than usual in all those events?

In summary: to me it looks like the muons come from the b-jets, together with some random fluctuation that got cherry-picked.

Certainly worth a check by the LHC experiments, but I would be very surprised if they see anything. Chances are good they looked already and saw nothing.

If the "LHC nightmare scenario" would indeed (God forbid) come to pass: is quantum entanglement an effect of quantum gravity at low energy scale? Can it be approached both experimentally and from theory, how?

Andreas Ringwald has an updated summary of his SMASH model on the ArXiv as well.

Besides being a bit ad-hoc, it also suffers from a bit of author bias. I would prefer to nail down the masses and numbers of the axions (none, one or many, wispy, fuzzy light or heavy) before developing specific models around it. Right now I am happy enough simulating axions and axion physics, and its relationship with the Higgs and gravitational anomalies.

About Résonaances

Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The posts are often spiced with sarcasm, irony, and a sick sense of humor. The goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...